Stereo expansion in audio systems is well known in the art and has been available for many years. In such systems, the right and left channel signals are processed in a manner wherein psychoacoustic effects makes it acoustically appear to the listener that the spatial separation of the loudspeakers is greater than the actual physical separation. Stereo expansion is described in, % inter alia, U.S. Pat. Nos. 4,495,637 of Bruney, 4,831,652 of Anderson et al., 5,208,493 of Lendaro et al., and 5,850,453 of Klayman et al., and e.g., was used in the RCA CTC 169 color television chassis of Thomson Consumer Electronics Inc., U.S.A.
An exemplary prior art stereo expansion circuit is shown in FIG. 1. wherein stereo expansion circuit 10 includes two operational amplifiers (opamps) 11 and 12. A left (L) channel signal is applied to the positive (non-inverting) input terminal of opamp 11 by way of an input line 13. A portion of the right (R) stereo channel is cross-fed to the left (L) channel opamp 11 at the inverting input terminal thereof. A similarly processed inverted portion of the left channel is cross-fed to the right channel opamp 12 at the inverting input terminal thereof.
Again referring to FIG. 1, the right and left channel signals are fed back by the respective resistors 16 and 19. The cross-channel cross-fed signals cause each channel's output to effect the output of the other channel. Specifically, because of the cross-feeding, the output signal on the left output line 15 of the opamp 11 is a L+X(L−R) signal while the output signal on the right output line 18 of the opamp 12 is a R+X(R−L) signal with the coefficient “X” being determined by the characteristics of the filter 22, and typically is between the values of one and two.
Filter 22 of the cross-feeding circuit includes a capacitor 24 and a resistor 25 which determine the crossover frequency at which cross-feeding occurs, i.e., very little coupling between channels at low frequencies, increased coupling as the frequency increases from about 150 Hz or 200 Hz, and full coupling at about 1 KHz to 3 KHz. The parallel combinations of resistor 16 and capacitor 27L, and resistor 19 and capacitor 27R, roll-off the frequency response of amplifiers 11 and 12 respectively to decrease the coupling between the channels through transmission gate 21 to virtually zero above 5 KHz, For the circuit of FIG. 1, the upper frequency break point for an exemplary channel is Fu=1/(2πcapacitor 27)(resistor 19)) and the lower frequency break point is Fl=1/(2π(capacitor 24)(resistor 25)). The frequency vs. amplitude response of the prior art circuit of FIG. 1 for a single channel input signal is shown in FIG. 2.
The bandlimiting of the crossfed signal to the midrange frequencies for the circuit shown in FIG. 1 often causes two undesirable artifacts of the perceived sound:                1. The overall sound tonality seems to have an increased midrange boost (FIG. 2) while the perceived center-stage image and tonality remain unchanged. Thus, the new perceived sound source, apparently located physically outside the stereo speaker pair locations, has a distinct midrange quality which appears in addition to the existing perceived image near center stage. If the listener moves to an off-axis location where the perceived stereo image deteriorates, the adding of the “expanded stereo” is heard as a midrange increase.        2. The midrange expanded-image program material can “drown out” a center stage performer particularly when the program material has a high content of difference signal (L−R) relative to sum signal (L+R) material.        